1. Field of the Invention
This invention relates to a low-birefringent organic optical component and a polymer with a spirobiindan structure. The low-birefringent organic optical component of this invention has excellent transparency, mechanical strength and heat resistance as well as a low birefringence, which is useful as, for example, a substrate for an optical disk, a pick-up lens, a plastic substrate for a liquid cell and a prism.
2. Description of the Related Art
Inorganic glasses have a number of excellent physical properties such as excellent transparency and a reduced optical anisotropy, and thus have been used in various fields. The glasses, however, have problems such as fragility due to their heavy weight and a poor productivity, leading to recent intensive attempts for developing a transparent polymer as a substitute for an inorganic glass.
A transparent polymer such as poly(methyl methacrylate) and polycarbonate has excellent transparency, mechanical properties such as shock resistance, processability and moldability, which has been, therefore, used in various applications such as transparent components of a car and a lens, as an alternative to an inorganic glass.
Meanwhile, an optical disk on which information such as sounds, images and texts is recorded and reproduced using a laser beam, has been rapidly extended in its use. In an optical disk used as an information recording medium, a laser beam passes through the disk body during its use. Thus, the disk is required to be optically transparent, and is strongly required to be optically homogeneous for reducing reading errors of an information. When using a conventional polymer such as polycarbonate and poly(methyl methacrylate), there occurs a problem that a residual stress generated by some factors such as temperature distribution, molecular orientation and volume variation near a glass-transition temperature generated by cooling and fluidizing processes of a resin during casting a disk substrate, may cause a birefringence when a laser beam passes through the disk substrate. Large optical heterogeneity due to the birefringence may become a fatal defect for an optical component such as an optical disk substrate because it may cause significant problems such as reading errors of a recorded information. Hence, an optical component, typically an optical disk substrate, is required to be made from a material with better optical characteristics than any of conventional polymers, e.g., a low birefringence and excellent transparency and heat resistance.
For dealing with the above problems, JP-A 63-314235 has disclosed a low-birefringent polycarbonate from a spiro compound such as a homopolymeric polycarbonate of spirobiindanol or a copolymeric polycarbonate of spirobiindanol and bisphenol-A. However, the former polycarbonate has a low birefringence, but is practically problematic due to its poor transparency and mechanical strength, while in the latter polycarbonate, increase of bisphenol-A improves transparency and mechanical strength, but increases the birefringence, leading to limiting its applications as an optical component. Thus, it has been strongly desired to solve these conflicting problems.
JP-A 3-162413 has suggested a polymer such as a polycarbonate having a spirobichroman structure as a material with a low birefringence. However, the polymer is also practically problematic due to its poor transparency and mechanical strength, although a homopolymeric polycarbonate of a spirobichroman derivative has a low birefringence. Furthermore, for a copolymeric polycarbonate of a spirobichroman derivative and bisphenol A, increase of bisphenol A causes increase of birefringence although it improves transparency and mechanical strength. Thus, it has been desired to solve these conflicting problems.
Polyimides are well known as an engineering plastic with a high heat resistance. Polyimides, however, have good heat resistance, but a high birefringence. For example, the polyimide described in JP-A 8-504967 may be used as an optical material, but has a birefringence of at least 0.01 level which is not adequately low. Furthermore, according to "PHOTOSENSITIVE POLYIMIDE--Fundamentals and Applications", edited by KAZUYUKI HORIE and TAKASHI YAMASHITA TECHNOMIC PUBLISHING COMP., p. 300 (1995), commercially available polyimides have a birefringence of at least 0.1; even a special fluorinated polyimide indicates a birefringence of 0.01 level. Thus, these may significantly improve heat resistance, but considerably limit their use as an optical component.
Aromatic polyimides are also known as an engineering plastic. However, optical properties, particularly a refractive index and a birefringence, have not been described very much for aromatic polyimides, and thus, substantially no data on the properties are available.
A polyimide can be prepared by reaction of a diamine with a tetracarboxylic dianhydride, and is excellent in some properties such as heat resistance, mechanical strength, chemical resistance, dimensional stability, incombustibility and electric insulation. Hence, polyimides have been extensively used in the fields of electric and electronic devices. In particular, they are anticipated to be more extensively and more largely used, in the fields requiring good heat resistance and electric insulation.
Polyimides have a high heat resistance and a high chemical resistance. It is advantageous in terms of its application to an optical component, while it is not necessarily advantageous in terms of processability. For example, Kapton or Vespel (DuPont) and Upilex (Ube Industries Ltd.), represented by formulas (A) and (B), are well-known polyimides.
Since these polyimides are insoluble, infusible and less processable, special and inefficient molding techniques such as compression and cutting or solution film casting of a polyamide acid as a precursor for a polyimide, have been used to prepare a molding or film. ##STR1##
To solve the above problems, Ultem (G. E.; U.S. Pat. Nos. 3,847,867 and 3,847,869), a polyetherimide represented by formula (C) has been developed. This polyetherimide can be subject to melt processing, and is soluble in a general solvent such as amides, phenols and halogenated hydrocarbons and thus has excellent processability as a solution. The polyetherimide, however, has a glass-transition temperature of about 215.degree. C., indicating an inadequate heat resistance. ##STR2##
The polyimides represented by formulas (D) and (E) (both are manufactured by Mitsui Toatsu Chemicals Inc.; JP-As 62-205124 and 2-18419) are known as those which has high heat resistance and can be subject to melt process. These polyimides have a glass-transition temperature of 250.degree. C. level, indicating a better heat resistance than the above polyetherimide (C) and can be subject to melt process, but has a poor solubility in a solvent and thus is difficult to be processed as a solution. ##STR3##
The heat-resistant adhesive polyimide represented by formula (F) is also known (JP-B 5-74637). The polyimide has the same level of heat resistance and thermal plasticity as those of formulas (D) and (E), as well as exhibits sufficient solubility in a solvent to be processable by melt processing and solution molding depending on selection of the group R; ##STR4## wherein R' represents a particular tetravalent aromatic group.
However, as aircraft and space instruments as well as electric and electronic devices will be advanced, higher heat resistance, i.e., a high glass-transition temperature, will be desired.
The above polyetherimide of formula (C) and the polyimide of formula (D) have a number of bending groups in their principal chain for improving melt fluidity and solubility. Increasing the number of bending groups generally results in increase of coefficient of linear thermal expansion of the resin itself. Since a resin with a low coefficient of linear thermal expansion is required in the field of electronic materials wherein fine processing is desired, the above polyimide of the prior art is not desirable.
Polyimides are anticipated to be more extensively and more largely used, in the fields of electric and electronic devices, especially in the fields requiring good heat resistance and electric insulation.
Recently, microelectronics have been markedly developed in electric and electronic fields. In particular, research and development of an insulating material for a multilayer circuit board has been extensively attempted. Polyimides, among the organic materials used in these fields, are preferably used as an insulating film because these are particularly excellent in heat resistance and dimensional stability and have a lower dielectric constant compared with an inorganic material. However, the dielectric constants of the commercially available polyimide resins are, for example, 3.6/1 kHz for a polyimide prepared from 4,4'-diaminodiphenyl ether and pyromellitic dianhydride (Trade Name: Kapton or Vespel); 3.5/1 kHz for a polyimide prepared from metadiaminobenzene and 3,3',4,4'-biphenyltetracarboxylic dianhydride (Trade Name: Upilex); and 3.7/1 kHz for a polyimide from 3,3'-diaminobenzophenone and 3,3',4,4'-benzophenonetetracarboxylic dianhydride (Trade Name: LARC-TPI).
Polyimide resins have been already used as an insulating material for a flexible printed board, but because of increasing integration of an electronic circuit, improvement in electric characteristics such as a lower dielectric constant has been desired. Specifically, an insulating material with a low dielectric constant of below 3.0, preferably about 2.8, is desired because, for example, for a large computer, using a multilayer circuit board makes it inevitable to transmit a signal at a high speed, while the signal transmission speed is inversely proportional to the dielectric constant of the board material, and thus the high dielectric constant of the board material causes delay in signal transmission, inhibiting speed-up. Since polyimides are used in an interlayer insulating film with a multilayer wiring structure, necessity for a polyimide with a low dielectric constant has been paid more attention in the light of the above reasons.
Teflon (DuPont) is a well-known resin with a low dielectric constant, while a variety of polyimides having a low dielectric constant have been developed as an engineering polymer having good properties such as heat resistance. Introduction of fluorine atoms or fluoroalkyl groups are known as a technique for giving a polyimide a low dielectric constant; specifically, such a low dielectric constant has become possible by using a fluorine-containing aromatic diamine or aromatic acid dianhydride as described by A. K. St. Clair et al. in NASA, U.S.A. (Polymeric Materials Science and Engineering, 59, 28-32 (1988)) and in EP 0299865. However, for the polyimides of the prior art, no descriptions are found with regard to molding processability such as melt process. Similarly, the polyimides described in U.S. Pat. No. 5,089,593 are difficult to be subject to melt processing because the diamine has an amino group at a para position to the bonding position and thus the overall polyimides are rigid and do not give melting fluidity when the acid anhydride is rigid.
Furthermore, colorless and transparent engineering plastics with a low dielectric constant have been extensively developed. The polycarbonate represented by formula (G) is well-known as a highly colorless and transparent resin. ##STR5##
The resin has a low glass-transition temperature of about 150.degree. C., indicating insufficient heat resistance.
The polyethersulfone (PES) represented by formula (H) is also well known as a transparent resin. ##STR6##
However, since the resin has a sulfone group which is highly water-absorptive, it is not preferable as a material for electric or electronic field where moisture should be eliminated.
Polyimides have been resins with dark reddish-brown to yellow, but a variety of polyimides which is colorless and transparent have recently been developed. For example, the polyimide represented by formula (I) has an excellent yellowness index (e.g., JP-A 1-182324). ##STR7##
However, this polyimide also has a problem of water absorptivity because of having a sulfone group like the above PES.
Furthermore, the above polyimides of NASA (A. K. St. Clair et al., Polymeric Materials Science and Engineering, 59, 28-32 (1988)) and EP 0299865 have a number of bending groups in their principal chain, and increase of the number of bending groups generally causes increase of the coefficient of linear thermal expansion of the resin itself. Since a resin with a low coefficient of linear thermal expansion is required in the field of electronic materials to which a fine processing is expected, the above polyimides of the prior art are not preferable.